By following a systematic cleaning and maintenance program in recent years, the Colorado River Aqueduct has been kept in condition to deliver substantially greater flows of water to the coastal plain of Southern California than it was originally designed to carry. The condition of the conduit surfaces is briefly described, as well as the annual cleaning operations designed to remove flow-impeding aquatic deposits from the walls of the conveyance system. An analysis of the variations in Manning's roughness coefficient n is presented for various components of the system. A comparison of such n values before and after mechanical cleaning of the aqueduct is given in summary form.